In recent years, a high-density multilayer printed wiring board is used in electronic equipment that is increasingly becoming smaller, thinner and lighter. The multilayer printed wiring board is becoming thinner, and a multilayer printed wiring board having an insulating layer thickness of 20 to 30 μm between an internal copper foil and an external copper foil is produced. Conventionally, a glass fiber woven cloth or nonwoven cloth or an organic fiber woven cloth or nonwoven cloth is used as a base material for a B-stage resin composition sheet for buildup lamination. However, there is a limitation on the making of a thin base material, so that resin layers can not be sufficiently formed on front and reverse surfaces of the base material. After laminate-molding, the base material is in contact with an internal copper foil and an external copper foil so that the migration resistance in the Z direction and the soldering heat resistance after moisture absorption are poor. Accordingly, there is a problem in reliability as a high-density multilayer printed wiring board.
Further, an adhesive sheet obtained by attaching a B-stage resin composition to a release film or a copper foil is used. However, when the adhesive sheet is used to produce a high-density multilayer printed wiring board having a small insulating layer thickness, the obtained multilayer printed wiring board is poor in reliability such as migration resistance in the Z direction. Further, it is also poor in electric characteristics and heat resistance so that it has a limitation in use as a high-density printed wiring board.
Further, concerning the production of a high-density multilayer printed wiring board, there is a method of producing a multilayer printed wiring board by using an additive process as a method for forming a fine circuit. An additive process multilayer printed wiring board using a conventional adhesive sheet which is obtained by adding a large amount of rubber into an epoxy resin and is not reinforced with base material, is poor in reliability such as migration resistance in the Z direction, particularly when the insulating layer thickness is small. Further, the above additive process multilayer printed wiring board is also poor in electric characteristics and heat resistance so that it has a limitation in use as a high-density printed wiring board.
In addition, when an adhesive sheet for a subtractive process or an adhesive sheet for an additive process, each of which sheet is not reinforced with a base material, is used on each surface of a thin internal layer board, a buildup-multilayered printed wiring board is poor in mechanical strengths such as bending strength and tensile strength and elastic modulus (hardness) and warps and distortions are apt to occur. Further, a thickness variance after molding is large, which causes defectives in an assembly stage and the like.